Contributed Talk - Splinter JungeAG

Monday, 12 September 2022, 16:10   (SFG 1010 / virtual JAG)

Probing the Galactic outer halo using kinematics and stellar abundances of distant RR Lyrae

Gustavo Medina 1, Camilla J. Hansen 2, Ricardo R. Muñoz 3, Eva K. Grebel 1, Kathy Vivas 4, Jeffrey Carlin 5, Clara Martinez-Vazquez 6
1 Universität Heidelberg, 2 Goethe University Frankfurt, 3 Universidad de Chile, 4 Cerro Tololo Inter-American Observatory, 5 Rubin Observatory Project Office, 6 Gemini Observatory

In the current cosmological paradigm, the Milky Way was formed hierarchically through the accretion of smaller systems, and imprints of these interactions are visible in the present-day orbital and chemical properties of Galactic halo stellar populations. RR Lyrae stars are ideal tracers of these merger events, as their status of old and precise distance indicators removes distance as one of the biggest limitations in the study of the outer halo. However, deriving radial velocities and spectroscopic metallicities of remote RR Lyrae is challenging, owing to their variability in short time-scales and the need for dedicated time at large aperture telescopes. In this contribution, I will present the spectroscopic analysis of 20 outer halo RR Lyrae (with distances between 15 and 165 kpc) in the context of the Halo Outskirts With Variable Stars (HOWVAST) survey, conducted using medium-resolution spectra from the MIKE spectrograph. In combination with Gaia DR3 data, we model the orbits of our stars considering the gravitational perturbation of the halo by the Large Magellanic Cloud, and determine atmospheric parameters and chemical abundance ratios (including alpha-elements, Fe, and neutron-capture elements) for a subsample of them. We search for associations between these distant RR Lyrae with known satellites and accretion events by studying their chemodynamics and speculating about their parent populations and origins (formed in-situ vs. accreted). We test the hypothesis that the accretion of sub-haloes largely contributes to the outer halo stellar populations, and report on the limitations of state-of-the-art distant halo RR Lyrae spectroscopic surveys. I will discuss the implications of our findings, and their impact on our preparation for the analysis of large samples of halo RR Lyrae in the upcoming era of large scale photometric and spectroscopic surveys.